1. Field of the Invention
The present invention relates to a graphite material.
2. Discussion of the Background
Since graphite materials are excellent in chemical stability, thermal resistance, processing characteristic, and the like, the materials have been used in many fields including an electrode for electric discharging, a jig for glass-sealing and brazing of electronic parts and an elastic body. Recently, with miniaturization of home electric appliances and automobile parts, precise processing to form thin ribs and grooves, thin pins, fine holes, and the like is performed to molds for use in die-casting and plastic molding. For the purpose of preparing such precise molds, an electrode for electric discharging comprising a graphite material which can be precisely processed has been needed.
For obtaining a precise form such as a thin rib by electric discharging using a graphite material as an electrode without breaking the electrode, it is necessary for the graphite material to have some degree of strength. Moreover, in order to enhance dimensional accuracy of a mold to be processed, it is important for the graphite material not to be deformed by heat and external force during electric discharging.
As a high-strength and high-density graphite material suitable for such an application, JP-A-1-97523 describes to use mesocarbon microbeads as a raw material. As another means for producing a high-density and high-strength graphite material, JP-A-4-240022 describes to mold mesocarbon microbeads having specific β resin content, ash content, water content, volatile content, fixed carbon, and average particle diameter as a raw material under cold press, and burning and graphitize it at a predetermined temperature. Since the graphite materials obtained by production processes described in JP-A-1-97523 and JP-A-4-240022 have high strength and high density, it is advantageous that the materials are difficult to break even when they are processed into a precise form such as a thin rib. The contents of JP-A-1-97523 and JP-A-4-240022 are incorporated herein by reference in their entirety.
Meanwhile, JP-A-6-144811 describes a carbonaceous coil spring in order to remedy a disadvantage of conventional springs such as a metal spring and a ceramic spring. That is, a metal spring has a large temperature dependency in the spring constant and thus is generally used at 200° C. or lower, and its thermal resistance is also limited to 600° C. and the strength rapidly decreases above the temperature. Moreover, the metal spring is poor in corrosion resistance against rust and chemicals. The thermal resistance of a ceramic spring is also limited to 1000° C. and the thermal shock resistance of the ceramic spring is poor. Since both of metal and ceramic have high specific gravity, it is disadvantageous that a device having the metal or ceramic spring incorporated therein has a large weight.
The method for obtaining the carbonaceous coil spring described in JP-A-6-144811 includes: forming an organic material capable of carbonization or an organic string body, which contains carbon fibers, graphite whiskers, graphite powders, amorphous carbon powders, or the like homogeneously dispersed therein and is highly reinforced, into a coil shape; subjecting it to a carbon precursor treatment as needed; carbonizing it through a heating treatment in an inert atmosphere; and covering the whole surface of the carbonized spring with a metal corresponding to a desired function. The carbonaceous coil spring has excellent thermal resistance and corrosion resistance even at a high temperature in the presence of oxygen and is expected to have high strength and reliability.